1. Field of the Invention
This invention relates to a spark ignition type in-cylinder injection internal combustion engine, which can inject fuel during its compression stroke to perform stratified combustion, and, in particular, to a spark ignition type in-cylinder injection internal combustion engine, which is suitably used as an automobile engine.
2. Description of Background Art
Recently in practical use is a spark ignition type in-cylinder injection internal combustion engine, which performs spark ignition by use of a spark plug and directly injects fuel into a cylinder. Such a spark ignition type in-cylinder injection internal combustion engine can improve both its fuel consumption performance and output performance by utilizing its characteristic features that its injection timing can freely be set and that the state of formation of air/fuel mixture can freely be controlled.
Namely, by injecting fuel during a compression stroke, the spark ignition type in-cylinder injection internal combustion engine can perform, due to stratified combustion, an operation (ultra-lean burn operation) in a state where fuel is quite lean (i.e., where the air/fuel ratio is much higher than a stoichiometric air/fuel ratio), and is equipped with an ultra-lean operation mode (compressed lean operation mode) as its combustion mode, thus allowing fuel consumption ratio to greatly improve.
On the other hand, it can perform a pre-mixture burn operation mainly by injecting fuel during an intake stroke, of course. In this case, by directly injecting fuel into a combustion chamber (within a cylinder), most of injected fuel can securely be burned within its combustion cycle, thus contributing to improving output as well.
It is also possible, in such a pre-mixture burn operation to set, as combustion mode, a lean operation mode (intake lean operation mode) for performing an operation in a state where fuel is lean (i.e., where the air/fuel ratio is higher than the stoichiometric air/fuel ratio) but richer than that in the ultra-lean mode, a stoichiometric operation mode (stoichiometric feedback operation mode) for performing feedback control based on information from an O.sub.2 sensor or the like so that the air/fuel ratio becomes the stoichiometric air/fuel ratio, and an enriched operation mode (open-loop mode) for performing an operation in a state where fuel is rich (i.e., the air/fuel ratio is lower than the stoichiometric air/fuel ratio).
Then, among these various kinds of operation modes, an appropriate mode is selected according to an operation state of the engine, i.e., state of engine speed and load, to control the engine.
In general, when the output required for the engine is small, i.e., when both engine speed and load are low, the compressed lean mode is selected in order to improve fuel consumption ratio; and as the engine speed and engine load increase from there, the intake lean operation mode, stoichiometric operation mode, and enriched operation mode are successively selected.
Meanwhile, in general as shown in FIG. 14(B), the higher the temperature or pressure in the combustion chamber is, self-ignition of fuel and knocking of the engine are more likely to occur, thus limiting improvement in compression ratio from the viewpoint of knocking elimination. In the spark ignition type in-cylinder injection internal combustion engine, by contrast, knocking is hard to occur because air inducted in the combustion chamber is cooled by injecting fuel from an early stage of an intake stroke, whereby compression ratio can be set higher.
On the other hand, it has been found that, when the compression ratio is set higher in the spark ignition type in-cylinder injection internal combustion engine, knocking occurs only for a short period of time upon starting in the case of a vehicle equipped with an automatic transmission (AT vehicle) in particular. When gasoline is used as fuel, this phenomenon is more remarkable with gasoline having a lower octane number (regular gasoline) than that having a higher octane number (premium gasoline).
It is presumed that knocking occurs in the AT vehicle upon starting due to a low speed and high load state generated at that time.
Namely, as shown in FIG. 15, upon starting the AT vehicle, as an accelerator is activated from an idle operation state, the intake air amount drastically increases in response to a rapid increase in throttle opening angle, thereby dramatically enhancing the engine load. By contrast, the engine rotational speed increases slower than the engine load, thus temporarily yielding a low-speed and high-load state (see region LH in FIG. 15) though for a short period of time.
Namely, when the engine operates at a low speed under high load, the stoichiometric operation mode or enriched operation mode is adopted. These modes attain an air/fuel ratio (on the order of 12 to 18), at which knocking is likely to occur, as shown in FIG. 14(A). Also, in this operation mode, while a large amount of fuel is injected into the combustion chamber from the first half of the intake stroke, a low speed state is temporarily generated as mentioned above, whereby it takes a long time for the injected fuel to be atomized, thus becoming easy to self-ignite. Consequently, knocking is very likely to occur in the low-speed and high-load state.
In a vehicle equipped with a manual transmission (MT vehicle), since the engine rotational speed rises during a delay in clutch operation, knocking upon starting occurs less frequently than in the AT vehicle.
As means against such starting knocks, the ignition timing may be retarded. Nevertheless, retarding control is limited in order to secure starting torque, thus making it difficult to sufficiently prevent starting knock from occurring.
Accordingly, further proposed as means against starting knock is, for example, setting an idle speed higher in the D range of the AT vehicle to suppress the degree of lowering the rotational speed upon starting and secure the starting torque.
When the idle speed is set higher, however, fuel consumption ratio consequently deteriorates.
An example of techniques for preventing knocks in the spark ignition type in-cylinder injection internal combustion engine is disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 7-189767. In this technique, fuel is injected a plurality of times, so that a uniform air/fuel mixture is formed at an earlier injection, and a spark is generated at a later injection in the vicinity of an ignition timing, whereby the uniform air/fuel mixture formed at the earlier injection is rapidly burned, in order to prevent knocks.
This technique is supposed to utilize fumigation which is one of knock preventing methods in diesel engines. Here, fumigation refers to a technique in which, in an intake stroke in a diesel engine, while fuel is being atomized or vaporized, it is mixed into intake air to such an extent that the air/fuel mixture does not cause self-ignition, whereby the retardation of ignition is reduced by a preflame reaction during a compression stroke, thus preventing knocks.
In order to adopt fumigation in a spark ignition type in-cylinder injection internal combustion engine, for example, fuel injection (earlier injection) may be performed at an intake stroke while preventing thus injected fuel from self-igniting, and ignition may be effected after fuel injection is performed at a compression stroke. Here, it is important for the fuel in the earlier injection to be kept from self-igniting. The above-mentioned publication, however, fails to specifically disclose how to prevent the earlier injected fuel from self-igniting. Rather, the earlier fuel injection amount is disclosed as being set greater, by which the earlier injected fuel is likely to self-ignite, thus failing to securely prevent knocking.
Also, while fumigation is effective in preventing knocks and increasing output, it disadvantageously increases the HC (hydrocarbon) amount in exhaust gas, which may result in problematic exhaust gas odor in diesel engines. In the case where an antiknock technique based on fumigation is adopted in a spark ignition type in-cylinder injection internal combustion engine, though its advantages are supposed to be utilized without yielding so much problems in terms of exhaust gas odor, it is necessary for such technique to be used under an appropriate condition in view of the overall performance of the engine. Nevertheless, the above-mentioned publication fails to fully disclose this point, thus leaving problems concerning such a control condition for preventing knocks.